Microcomputers generally comprise a processor and memory and may operate in accordance with a sequence of instructions derived from a stored program. The instructions may include a so-called "function" selected from a set of selectable functions and which the define the operation which is carried out by the processor in response that instruction. Processors may wish to communicate messages between different processes. Difficulties have previously arisen in effecting rapid and satisfactory communication between processes on the same microcomputer and between different microcomputers. Furthermore difficulties have arisen in scheduling and descheduling processes to permit a processor to share its time between a plurality of concurrent processes.